This invention relates to devices which measure mechanical strain and more particularly to devices which measure mechanical strain in bolts or studs.
The proper assembly of bolted joints on high pressure devices such as turbine housings requires high clamping forces at the bolted joints. For example, certain turbine assembly clamp bolts must be pre-stressed to approximately 45,000 psi for adequate clamping force. This is equivalent to an increase in length of 0.0015 inches per inch of effective bolt length.
Three basic methods can be used to measure bolt strain or elongation when a standard hydraulic wrench is used for tightening. These methods are the heating method, torque method and direct measurement method.
In the heating method, a heating rod is inserted into a central axial hole in the bolt. Then the bolt is heated until a specific thermal expansion is achieved. At that time a nut can be tightened snugly and the bolt is allowed to cool. The time required to heat each bolt is long and heater reliability is low. Many heater sizes are required to accommodate the various bolt lengths and bolt sizes.
In the torque method, bolts are tightened to a specific torque which has been calculated or experimentally determined to equate to the desired elongation. However, this does not provide for direct control over bolt tension and is unacceptable where bolt tension is critical.
A variation of the torque method involves pretorquing a nut to seat it on the housing, then the nut is rotated through predetermined degrees of rotation, using the bolt threads as a micrometer. The bolt is stretched a predetermined amount corresponding to a specific tension. This method requires a specially designed wrench with precision instrumentation to control the degrees of rotation. Therefore, it is too complex for many applications.
A second variation of the torque method involves the simultaneous monitoring of torque rate and degrees of rotation. The control scheme for this method depends on detection of the bolt yield point, making it undesirable for stress levels below the yield stress.
Direct measurement of the change in bolt length during tightening can be accomplished in several ways. Ultrasonic techniques can be employed, using either a pulse echo or standing wave method. These methods are relatively complex, require a certain degree of operator skill and are subject to transducer mounting problems.
Built-in measuring rods which are located within the central bore of the bolt have been suggested. Bolt elongation would be determined by examining the amount of rod protruding from the end of the bolt or by measuring the location of the end of the rod using an eddy current technique. Each of these methods requires a special bolt with a built-in rod.
Measuring rods have been used in conjunction with conventional depth micrometers to measure bolt length. This method requires that an initial measurement be taken. Then the bolt is tightened and the wrench is removed so that a second measurement can be taken. This method is rather slow due to the necessity of removing the wrench to make repeated measurements. Nut rotation can be observed on the first few bolts, then nut rotation is used for subsequent bolts. Consistency suffers when this method is used.
U.S. Pat. No. 3,943,819 issued Mar. 16, 1976 to Charron, discloses a direct-reading tension-indicating device. The device includes a tubular sleeve and a test rod which extend coaxially into the bore of the bolt being measured. The sleeve and rod are secured at longitudinally spaced positions within the bolt. Both the sleeve and the rod have a free end adjacent to the open end of the bore. Tensile stress within the intermediate region of the body of the bolt is measured by measuring relative longitudinal displacement between the end faces of the sleeve and rod at their free ends. U.S. Pat. No. 3,877,326 issued Apr. 15, 1975 to Kock et al., and U.S. Pat. No. 3,837,694 issued Sept. 24, 1974 to Frisch et al., both disclose bolt tensioning devices which include continuous display of bolt elongation. Each device utilizes a measuring rod which extends into a bore of the bolt being stressed. Bolt elongation is then determined by measuring the displacement between the end of this measuring rod and the top of a bolt or a bolt extension assembly. In the devices disclosed in each of the three cited patents, the end of the measuring or test rod within the bolt bore is used as a reference point for determining bolt elongation. Therefore, the length of the measuring or test rod is specific to the particular bolt and/or tensioning apparatus being used.
The present invention provides a bolt stretch measurement device which utilizes a measuring rod, the length of which is independent of the bolt being measured. Therefore, bolts and studs of various lengths can be measured with the same instrument. A bolt strain measurement device constructed in accordance with the present invention includes a bolt extension tube which makes contact with the bolt being measured at a point near one end of the bolt. The bolt extension tube extends into a central bore of the bolt being measured to provide support for the measurement device during tightening of the bolt. A coaxial measuring rod extends through the bolt extension tube into the central bore in the bolt being measured and comes in contact with the bolt at a point near the opposite end of the bolt. A slide assembly is slidably disposed along the measuring rod and capable of being clamped to a point on the measuring rod above the end of the bolt extension tube. When the bolt is strained by tightening, the bolt extension tube will move with respect to the measuring rod. With the slide assembly clamped, displacement between the slide assembly and the bolt extension tube will be proportional to bolt elongation. Means for measuring this displacement is provided and can supply a signal for a display of the displacement value or for control of the bolt tightening mechanism.